Rene W. Goosmann

Modeling optical and UV polarization of AGNs - II. Polarization imaging and complex reprocessing

Context. The innermost parts of active galactic nuclei (AGNs) are believed to be comprised of several emission and scattering media coupled by radiative processes. These regions generally cannot be spatially resolved. Spectropolarimetric observations give important information about the reprocessing geometry. Aims. We aim to obtain a coherent model of the polarization signature resulting from the radiative coupling between the components, to compare our results with polarimetry of thermal AGNs, and thereby to put constraints on the geometry. Methods. We used a new public version of stokes, a Monte Carlo radiative transfer code presented in the first paper of this series. The code has been significantly improved for computational speed and polarization imaging has been implemented. The imaging capability helps to improve understanding of the contributions of different components to the spatially-integrated flux. We coupled continuum sources with a variety of reprocessing regions such as equatorial scattering regions, polar outflows, and toroidal obscuring dust and studied the resulting polarization. We explored combinations and computed a grid of thermal AGN models for different halfopening angles of the torus and polar winds. We also considered a range of optical depths for equatorial and polar electron scattering and investigated how the model geometry influences the type-1/type-2 polarization dichotomy for thermal AGNs (type-1 AGNs tend to be polarized parallel to the axis of the torus while type-2 AGNs tend to be polarized perpendicular to it). Results. We put new constrains on the inflowing medium within the inner walls of the torus. To reproduce the observed polarization in type-1 objects, the inflow should be confined to the common equatorial plane of the torus and the accretion disk and have a radial optical depth of 1 60 ◦ to match the expected level of perpendicular polarization. If outflows are collimated by the torus inner walls, they must not be optically thick ( τ 0.3.

Modeling optical and UV polarization of AGNs - III. From uniform-density to clumpy regions

A growing body of evidence suggests that part of, if not all, scattering regions of active galactic nuclei (AGNs) are clumpy. Hence. in this paper, we run radiative transfer models in the optical/UV for a variety of AGN reprocessing regions with different distributions of clumpy scattering media. We use the latest version of the Monte Carlo code STOKES presented in the first two papers of this series to model AGN reprocessing regions of increasing morphological complexity. We replace previously uniform-density media with up to thousands of constant-density clumps. We couple a continuum source to fragmented equatorial scattering regions, polar outflows, and toroidal, obscuring dust regions and investigate a wide range of geometries. We also consider different levels of fragmentation in each scattering region to evaluate importance of fragmentation for the net polarization of the AGN. We find that, in comparison with uniform-density models, equatorial distributions of gas and dust clouds result in grayer spectra, and show a decrease of the net polarization percentage at all lines of sight. The resulting polarization position angle depends on the morphology of the clumpy structure, with extended tori favoring parallel polarization while compact tori produce orthogonal polarization position angles. In the case of polar scattering regions, fragmentation increases the net polarization unless the cloud filling factor is small. A complete set of AGN models constructed from the individual, fragmented regions is investigated. Our modeling shows that the introduction of fragmented dusty tori significantly alters the resulting net polarization of an AGN. Comparison of our models to polarization observations of large AGN samples greatly favors geometrically compact clumpy tori over extended ones.

X-ray polarimetry as a new tool to discriminate reflection from absorption scenarios — predictions for MCG-6-30-15

We present modelling of X-ray polarization spectra emerging from the two competing scenarios that are proposed to explain the broad Fe Kα line in the Seyfert 1 galaxy MCG-6-30-15. The polarization signature of complex absorption is studied for a partial covering scenario using a clumpy wind and compared to a reflection model based on the lamppost geometry. The shape of the polarization percentage and angle as a function of photon energy are found to be distinctly different between the reflection and the absorption cases. Relativistic reflection produces significantly stronger polarization in the 1–10 keV energy band than absorption. The spectrum of the polarization angle adds additional constraints: in the absorption case it shows a constant shape, whereas the relativistic reflection scenario typically leads to a smooth rotation of the polarization angle with photon energy. Based on this work, we conclude that a soft X-ray polarimeter onboard a small X-ray satellite may already discriminate between the absorption and the reflection scenarios. A promising opportunity may arise with the X-ray Imaging Polarimetry Explorer mission, which has been proposed to the European Space Agency in response to a small-size (S-class) mission call due for launch in 2017.

A structure for quasars under the scope of polarization – I. The UV/optical polarization dichotomy of type-1 and type-2 AGN

We present UV/optical spectropolarimetric modelling of the phenomenologically-based structure for quasars proposed by Elvis (2000). In this first paper of a series, we explore the continuum polarisation emerging from radiatively accelerated and bent winds that were vertically launched from the accretion disc in an active galactic nucleus (AGN). We simulate the radiative transfer occurring in Thomson scattering and dust extinction media over a range of morphological parameters and optical depths of the wind. We demonstrate that the wind geometry proposed by Elvis with a phenomenologically-derived bending angle of theta = 60deg still underestimates the observed optical polarisation percentage of type-1 and type-2 AGN and does not yet reproduce the expected dichotomy of the polarisation position angle. To recover the observed polarisation properties, a smaller bending angle and some amount of dust shielding in the equatorial region should be considered. A two-phase outflow is found to generate both the observed polarisation dichotomy and acceptable levels of polarisation degree if the wind has a bending angle theta = 45deg, and the conical shells have a half-openingangle of 3deg < delta_theta < 10deg. The absorbing dust column at the wind base should be in the range of 1 < tau_dust < 4 (tau being integrated over 2000 - 8000 Angs). Straightforward observational tests from spectropolarimetry and from determining the number density of different AGN types can be performed to further constrain the wind geometry.

Off-axis irradiation and the polarization of broad emission lines in active galactic nuclei

Abstract The stokes Monte Carlo radiative transfer code has been extended to model the velocity dependence of the polarization of emission lines. We use stokes to present improved modeling of the velocity-dependent polarization of broad emission lines in active galactic nuclei. We confirm that off-axis continuum emission can produce observed velocity dependencies of both the degree and position angle of polarization. The characteristic features are a dip in the percentage polarization and an S-shaped swing in the position angle of the polarization across the line profile. Some differences between our stokes results and previous modeling of polarization due to off-axis emission are noted. In particular we find that the presence of an offset between the maximum in line flux and the dip in the percentage of polarization or the central velocity of the swing in position angle does not necessarily imply that the scattering material is moving radially. Our model is an alternative scenario to the equatorial scattering disk described by Smith et al. (2005). We discuss strategies to discriminate between both interpretations and to constrain their relative contributions to the observed velocity-resolved line and polarization.

X-ray polarimetric signatures induced by spectral variability in the framework of the receding torus model

Obscuring circumnuclear dust is a well-established constituent of active galactic nuclei (AGN). Traditionally referred to as the receding dusty torus, its inner radius and angular extension should depend on the photo-ionizing luminosity of the central source. Using a Monte Carlo approach, we simulate the radiative transfer between the multiple components of an AGN adopting model constraints from the bright Seyfert galaxy NGC 4151. We compare our model results to the observed near-IR to UV polarization of the source and predict its X-ray polarization. We find that the 2-8 keV polarization fraction of a standard AGN model varies from less then a few percent along polar viewing angles up to tens of percent at equatorial inclinations. At viewing angles around the type-1/type-2 transition the X-ray polarization variability differs between a static or a receding torus scenario. In the former case, the expected 2-8 keV polarization of NGC 4151 is found to be 1.21% +/- 0.34% with a constant polarization position angle, while in the later scenario it varies from 0.1% to 6% depending on the photon index of the primary radiation. Additionally, an orthogonal rotation of the polarization position angle with photon energy appears for very soft primary spectra. Future X-ray polarimetry missions will be able to test if the receding model is valid for Seyfert galaxies seen at a viewing angle close to the torus horizon. The overall stability of the polarization position angle for photon indexes softer than {\Gamma} = 1.5 ensures that reliable measurements of X-ray polarization are possible. We derive a long-term observational strategy for NGC 4151 assuming observations with a small to medium-size X-ray polarimetry satellite.

The thermal instability of the warm absorber in NGC 3783

We model the observed X-ray spectral continuum shape, ionic column densities, and absorption measure distribution (AMD) of the warm absorber in the Seyfert galaxy NGC 3783. We assume a photo-ionized medium with a uniform total (gas+radiation) pressure. The irradiation causes the wind to be radiation pressure compressed (RPC). We compare the observational characteristics derived from the 900 ksec Chandra observation to radiative transfer computations in pressure equilibrium using the radiative transfer code TITAN. We explore different values of the ionization parameter xi of the incident flux and adjust the hydrogen-equivalent column density, N_H0 of the warm absorber to match the observed soft X-ray continuum. We derive theoretical column densities for a broad range of ionic species of iron and neon and an AMD that we compare to the observations. We find an extension of the degeneracy between xi and N_H0 for the constant pressure models previously discussed for NGC 3783. Including the ionic column densities of iron and neon in the comparison between observations and data we conclude that a range of ionization parameters between 4000 and 8000 ergs cm/s is preferred. For the first time, we present theoretical AMD for a constant pressure wind in NGC 3783 that correctly reproduces the observed level and is in approximate agreement with the observational appearance of an instability region. Using a variety of observational indicators, we confirm that the X-ray outflow of NGC 3783 can be described as an RPC medium in pressure equilibrium. The observed AMD agrees with a uniformly hot or a uniformly cold thermal state. The measured ionic column densities suggest that the wind tends to the uniformly cold thermal state. The occurrence of thermal instability in the warm absorber model may depend on the computational method and the spatial scale on which the radiative transfer is solved.

AGN black hole mass estimates using polarization in broad emission lines

The innermost regions in active galactic nuclei (AGNs) were not being spatially resolved so far but spectropolarimetry can provide us insight about their hidden physics and the geometry. From spectropolarimetric observations in broad emission lines and assuming equatorial scattering as a dominant polarization mechanism, it is possible to estimate the mass of supermassive black holes (SMBHs). We explore the possibilities and limits and to put constraints on the usage of the method for determining SMBH masses using polarization in broad emission lines by providing more in-depth theoretical modeling. Methods. We use the Monte Carlo radiative transfer code STOKES for exploring polarization of Type 1 AGNs. We model equatorial scattering using flared-disk geometry for a set of different SMBH masses assuming Thomson scattering. In addition to the Keplerian motion in the BLR, we also consider cases of additional radial inflows and vertical outflows. We model the profiles of polarization plane position angle, degree of polarization and total unpolarized line for different BLR geometries and different SMBH masses. Our modeling confirms that the method can be widely used for Type-1 AGNs when viewing inclinations are between 25 and 45 degrees. We show that the distance between the BLR and scattering region (SR) has a significant impact on the mass estimates and the best mass estimates are when the SR is situated at the distance 1.5-2.5 times larger than the outer BLR radius. Our models show that if Keplerian motion can be traced through the polarized line profile, then the direct estimation of the mass of the SMBH can be performed. When radial inflows or vertical outflows are present in the BLR, this method can be applied if velocities of the inflow/outflow are less than 500 km/s. We find that models for NGC4051, NGC4151, 3C273 and PG0844+349 are in good agreements with observations.

On the Interplay Between Radial and Angular Reflection Emissivity from the Black Hole Accretion Disc

Accretion processes around relativistic compact objects, such as black holes or neutron stars, can be well studied through X-ray spectroscopy. The disc reflection spectra detected in observations of several active galactic nuclei and X-ray binaries in our and nearby galaxies suggest high steepness of the radial emissivity. This can be primarily caused by compactness of the illuminating radiation. In our recent paper [1], we showed that the measurement of the steep radial emissivity index might also be over-estimated by ignoring the radial ionisation structure and the proper angular-emissivity law, which is non-trivial in the fully relativistic regime. In this paper, we demonstrate the interplay of the angular and radial emissivity. Employing an improper angular emissivity in the reflection models leads to over-estimated values for the black-hole angular momentum and the radial-emissivity index (by about 10 %).

Role of the emission angular directionality in the spin determination of accreting black holes

We discuss the method to determine the spin of black holes by employing the broad relativistic iron line profiles in the X‐ray domain. The precision of the spectral fitting procedure could be compromised by an inappropriate account for the angular distribution of the disc emission. We perform radiation transfer computations of an X‐ray irradiated disc atmosphere to constrain the directionality of the outgoing X‐rays in the 2–10 keV energy band. We study how sensitive the spin determination is to the assumptions about the intrinsic angular distribution of the emission.